Opposing effects of nitroxide free radicals in Escherichia coli mutants deficient in DNA repair

Nitroxide free radicals have been previously shown to function as superoxide dismutase (SOD) mimics and to protect bacterial and mammalian cells against oxidative damage, particularly from superoxide and hydrogen peroxide. Although nitroxides are generally considered to be non-toxic nor mutagenic, there is no agreement regarding their potential adverse effect, Some toxic effects were observed upon using high concentration of six-membered ring derivatives. Conflicting evidence has also been reported regarding the mutagenic activity of nitroxides toward Salmonella typhimurium. It was also demonstrated that nitroxides exert two opposing effects on exonuclease III deficient cells of Escherichia coli upon exposure to naphthoquinones. The attempts to use nitroxides as contrast agents in nuclear magnetic resonance imaging (MRI) and as a new class of anti-oxidants underscore the need to examine their potential adverse effects. Since nitroxides protected xthA cells from DNA scission caused by H2O2, it was anticipated that they would provide even greater protection for recA DNA repair-deficient cells of E, coli, which are more sensitive to H2O2-induced oxidative stress. The results of the present study showed that: (a) nitroxides exert bactericidal and bacteriostatic effects on recA but not on xthA or wild-type E. coli K12 cells; (b) nitroxides and H2O2 act synergistically on recA cells, both under aerobic and hypoxic conditions; (c) the nitroxide-induced toxicity in recA cells and the synergistic effect with H2O2 were not accompanied by a decrease in the cellular level of reduced glutathione; (d) TEMPAMINE protected against DNA scission induced by H2O2 and 1,10-o-phenanthroline chelate of Cu(II) in xthA cells, but potentiated DNA double-strand breakage in recA cells.